Zhang Xuhu, born in 1966, is mainly engaged in the research of metal matrix composites. Because of its high specific strength, specific modulus and good heat resistance, B/Al composites play an important role in the aerospace industry.
Up to now, B/Al is a metal matrix composite (MMC) with excellent performance and the most mature production technology. Internationally (mainly the United States and Russia) have entered the practical engineering test application stage from the laboratory research stage, and have carried out extensive application tests on various aerospace aircraft. For example, in the United States, the B/Al tube with Ti-6Al-4V alloy end ring and end joint made of boron fiber strong 6061 aluminum alloy is used on the space shuttle of the spacecraft, and the main skeleton and rib truss strut, skeleton stabilization bracket and brake are fabricated. Support column, etc. In the first three space shuttles, 243 of these B/Al pipe components were installed, and the total mass of the B/Al pipes was 150 kg. These composite pipe truss members met all the performance requirements of the space shuttle's actual flight. In the middle fuselage structure, compared with the original aluminum alloy extrusion, it can lose 145kg, which is equivalent to a 44% reduction in quality. 1. Russia is not only in the research of B/Al composites, but also in production and application. The world's leading position. Russia has developed a bracket that can simultaneously place three satellites with B/Al composite pipe. If the structure uses titanium alloy, it needs 150kg~160kg, and B/Al is 100kg. The structure has good thermal stability and has achieved significant reduction. The weight effect sample number can be seen from Table 2, and the average tensile strength reaches 1 rate is controlled within 10%. The average compressive strength is 2510 MPa. Although the dispersion is large, the minimum compressive strength is also 2. From these data, it can be seen that the composite quality of the B/Al composite pipe used in this test is good. The metallographic structure and fracture of the tensile specimen are respectively. It can be seen that the composite material is compacted, free from defects, and the fibers are arranged in an orderly manner. It can be seen that the fracture is a mixed fracture model (M-fractuie) with both cumulative fractures (C-fracture) and non-cumulative fractures (Nc*ractuie). In some areas, the fibers were pulled out, and in the other area of ​​the fracture, no fibers were pulled out and it was flat. This shows that the composite effect is good and the interface bonding strength is moderate.
The strength of the bond, the pull-out type of fracture corresponds to a weaker interface strength. Theoretical analysis shows that under the action of external force, the composite material firstly produces microcracks on the weaker aluminum matrix and some weak parts of the fiber. When the microcracks on the aluminum matrix extend to the B/Al interface, there may be two kinds of Happening. If the interface is strong, the stress concentration at the crack tip is sufficient to break the fiber, and the crack propagates forward through the fiber, thus forming a flat fracture. If the interface is weakly combined, when the crack tip reaches the B/Al interface, the interface will be debonded, and the crack will extend longitudinally along the fiber surface until it reaches the weaker part of the fiber or there is a microcrack to achieve the microcrack penetration. This forms a pull-out type fracture. The mechanical properties of composites fractured by cumulative or non-cumulative fractures are not very good. Only when the interface is suitable, composites broken in mixed fracture mode will have better mechanical properties. Observed from the fracture morphology, it also shows that the composite effect of the B/Al composite pipe is good, and the interface bonding strength is moderate. Further observations can also be found that the fracture morphology of boron fiber mostly has a tapered fracture, and there are also a few flat fractures and fractures, which may be related to the defects of the fiber itself, but the damage phenomenon observed from the axial compression failure test. Analysis, the pipe structure has two weak areas, one is the large stress concentration at the joint of the pipe and the aluminum joint, and the other is that the joint of the pipe forming die forms a weak zone of strength, so that the compression failure of the pipe appears in the See the two areas to see that the structure and molding process of the tube still needs to be improved.
4 Conclusions (2) The tensile fracture model is a mixed fracture model, indicating that the composite effect is good and the bonding strength of the interface is moderate.
(3) The study on the failure mode of the pipe is considered to be the weak zone of the axial compression failure at the joint between the pipe and the joint and the joint formed during the pipe forming.
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